CHEMOIMMUNOTHERAPY: Patients receive chemoimmunotherapy comprising rituximab* intravenously (IV) over 4-6 hours on day 1, methotrexate (MTX) IV over 4 hours on day 2, cyclophosphamide IV over 2 hours, doxorubicin hydrochloride IV, and vincristine sulfate IV on day 3, and prednisone orally (PO) on days 3-7. Beginning 24 hours after completion of MTX, patients receive leucovorin calcium IV every 6 hours until blood levels of MTX are in a safe range. Patients also receive filgrastim (G-CSF) subcutaneously (SC) once daily (QD) beginning on day 3 and continuing until blood counts recover. Beginning no sooner than day 22, but no later that day 29 of the first course, patients receive a second course of chemoimmunotherapy as above. Patients with > 15% persistent bone marrow involvement may receive a third course of chemoimmunotherapy. Patients are restaged and those with progressive disease are removed from therapy.